In a current-mode controlled DC/DC converter, a controllable switch is coupled to an inductor to generate a periodically changing inductor current through the inductor. An outer voltage regulation loop comprises a current-mode controller that subtracts the output voltage of the converter from a reference voltage to supply an error signal that is processed to obtain a control signal. This control signal may be used as a set level for the peak current in the inductor. The processing usually comprises a PI or a PID controller, which receives the error signal and supplies the control signal. Therefore, often, this processing is also referred to as a controller. An inner current regulation loop switches off the controllable switch when a sense signal that is representative for the inductor current reaches the set level. Thus, the set level, which depends on the difference between the output voltage level and the reference voltage level, determines a peak current level of the current through the inductor. Many options to determine this sense signal are known. For example, the sense signal may be obtained with a current transformer, or as a voltage over an impedance in series with the inductor, this series impedance may be in the main current path of the switch.
Usually, the switch is switched on by a clock pulse generated by an oscillator. The on-time of the switch is the period of time between the instant the switch is switched on by the clock pulse and the instant the inductor current reaches the set level. The off-time of the switch is the period in time between the instant the inductor current reaches the set level and the next clock pulse. The repetition period is the sum of the on-time and the off-time. In a buck converter, during the on-time, the switch connects the inductor between an input voltage and the output and the inductor current increases. The input voltage may be supplied by a battery. During the off-time, another switch connects the inductor between the output and ground and the inductor current decreases. The topology of other current-mode controlled DC/DC converters, such as for example, boost, buck-boost, Cuk converters, is also well known.
Usually, slope compensation is required to damp the disturbances in the inductor current. The slope compensation is obtained by varying the set level as a function of time during the repetition period. Often, the current-mode controller either subtracts a sawtooth, a parabola, or a piecewise linear slope compensation signal from the control signal to obtain a slope compensated control signal. Now, this slope compensated control signal is used as the set level, and thus, the off-period starts at the instant the peak-current through the inductor reaches the level of the slope compensated control signal.
In some applications, such as for example in telecom systems, the reference voltage is varied to obtain a varying output voltage which fits the actual transmission power required. It is important that the output voltage of the power converter tracks the variations of the reference voltage optimally. It is a drawback of the known current-mode controlled DC/DC converter that its speed of reacting on a variation of the reference voltage is not optimal.
U.S. Pat. No. 6,611,131 discloses such a current-mode switching regulator. As prior art, a current-mode switching regulator is discussed in which a voltage clamp is present across the integrating capacitor of the I-controller. This voltage clamp limits the control voltage present at the integrating capacitor. Thus, also the inductor current will be limited. It is further disclosed that this prior art solution has the drawback that the actual value to which the inductor current is limited depends on the slope compensation. Therefore, U.S. Pat. No. 6,611,131 proposes to use a voltage clamp which clamps the voltage at an output of a buffer which buffers the voltage across the integrating capacitor. The voltage at which the output of the buffer is limited depends on the slope compensation. Now, the level at which the current is limited depends less on the slope compensation signal. However, this solution has the drawback that the integrating control loop is not closed during the limiting action and consequently, the voltage over the integrating capacitor drifts away.